1. Field of the Invention
This invention generally relates to optical fiber, and particularly relates to optical fiber having increased power handling capacity due to the suppression of stimulated Brillouin scattering (SBS), and to a method for making said fiber and fiber preforms therefor.
2. Description of Related Art
Stimulated Brillouin scattering (SBS), an optical nonlinearity, limits the maximum optical power throughput of optical fiber transmission systems. As input power increases above what is known as the threshold power, the power that can be transmitted along the optical fiber reaches an upper limit. Any additional input power to the fiber scatters in the backward direction due to interaction with acoustic phonons rather than propagating in the forward, launch direction as a higher power signal. Thus SBS, as it is called, reduces the signal to noise ratio at the receiver and can cause the transmitter to become unstable due to the entry of reflected light. Moreover, the increasing use of optical amplifiers, solid state Nd:YAG lasers, and external modulation at ever increasing data rates over longer and longer distances all combine to exacerbate SBS in both digital and CATV applications.
SBS is an interaction of optical photons with acoustic phonons of the glass matrix. Techniques suggested in the literature to increase the threshold power, minimize the detrimental effects of SBS, and increase the power handling capacity of the fiber rely, e.g., on broadening either the photon energy spectrum of the source or the phonon energy spectrum of the glass to reduce the efficiency of the interaction. One reported method proposes changing the refractive index profile along the length of the fiber (axial direction) by varying the background fluorine concentration. Another proposes wrapping the fiber around a central rod to induce stress to change the energy distribution of acoustic phonons. Some disadvantages of changing the index of refraction along the axial direction of the fiber, and tight fiber wrapping, include undesirable changes in other fiber properties and increased fatigue which impacts fiber lifetime. Wada, et al, Suppression of stimulated Brillouin scattering by intentionally induced periodical residual-strain in single-mode optical fibers, in Proceedings of European Conference on Optical Comm. 1991, paper MoB1, propose applying draw tension to induce a periodical residual strain along the fiber length that broadens the phonon energy distribution and reduces the SBS interaction. This reference discloses a step-index fiber having a SiO2 core and a Fxe2x80x94SiO2 cladding. Because the F-doped cladding has a lower viscosity, the draw tension is mainly applied to the core resulting in a deviation of the residual strain as a function of draw tension. The residual strain broadens the effective gain linewidth of the fiber by continuously shifting the central frequency of the Brillouin gain spectrum. This reference indicates that draw tension-induced SBS suppression in germanium-doped silica core/undoped silica cladding single-mode fiber is negligible. A more recent publication, Headly et al., OFC ""97, Paper WL25, Tech. Digest, describes a modest 3 dB increase in threshold power for this type of fiber for optimized draw tension profile and reported maximum allowable draw tension variation.
Accordingly, the invention is directed to an optical fiber having increased power handling capacity, and to a fiber preform from which such a fiber can be drawn, having a radially nonuniform coefficient of thermal expansion (CTE) and viscosity profile, which imparts a permanent differential stress profile through the visco-elastic and thermal-elastic properties of the resultant glass structure. In turn, the stress profile alters the local density to broaden the range of acoustic energies (or velocities) which acts to suppress SBS and increase the power handling capacity of the fiber. In addition, the invention relates to a method for producing such fiber and preform, and to a method for enhancing the SBS suppression effect of the permanent differential stress in the fiber.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus and method particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, an embodiment of the invention describes an optical waveguide fiber having increased power handling capacity comprising a core having radially nonuniform CTE and viscosity profiles in a part of the core (hereinafter, the xe2x80x9cdoped regionxe2x80x9d), between the center and an outermost region of the core; and a cladding surrounding the outermost region of the core. In an aspect of this embodiment, the part of the core having the radially nonuniform CTE and viscosity profiles includes immediately adjacent annuli of alternating higher and lower CTE""s and viscosities provided by dopant layers of different compositions (i.e., providing differing mechanical structure). The fiber has a substantially constant and/or uniform effective refractive index profile along its propagation axis (i.e., in the axial direction). In another aspect of this embodiment, annular regions (layers) consisting essentially of SiO2 doped with GeO2, GeO2+P2O5, or GeO2+F, will exhibit a higher CTE and, in most cases, a lower viscosity, than adjacent layers consisting essentially of SiO2 or SiO2+F. In a further aspect of this embodiment, the doped region is substantially located at a radial distance from the center of the core where the value of the area-averaged optical power peak is a maximum, the peak being proportional to |E|2rdr, where (E) represents the electric field, (r) is the radius of the fiber core, and (dr) is the differential radius.
In another embodiment, the invention describes a fiber precursor, referred to hereinafter as a (fiber) preform, from which an optical waveguide fiber having an increased power handling capacity can be drawn. The preform includes a core region having radially nonuniform CTE and viscosity profiles in a part of the core region between the core center and an outermost region of the core (doped region), and a cladding composition surrounding the outermost region of the core.
In an aspect of this embodiment, the doped region includes immediately adjacent annular compositional layers of differing CTE""s and viscosities provided by selected dopants. The preform has a substantially constant and/or uniform effective refractive index profile in the axial direction. As it is well known to those skilled in the art that the optical and compositional characteristics of a fiber mimic those characteristics of the preform from which it is drawn, the preform core region will likewise have layers of SiO2 doped with GeO2, GeO2+P2O5, or GeO2+F, alternating with layers of SiO2 or SiO2+F to provide the radially nonuniform profiles of CTE and viscosity. Although, in each of the embodiments described above, the core modifying dopants include phosphorous and fluorine, other dopants that provide a similar effect will be known to those skilled in the art.
Preferred layer compositions and dopant levels will be dictated by the desired interaction of the thermal-elastic and visco-elastic-induced stress in the fiber. The resultant radial strain, xcex94L/L (where L=the thickness of a particular layer) at the interfaces of the alternating layers, should be at least  greater than 0.001, and preferably  greater than 0.002, and will be upper bound by the mechanical fatigue of the glass.
A further embodiment of the invention provides a method for making an optical waveguide fiber having increased power handling capacity, involving the steps of providing a fiber preform having a core region with radially nonuniform CTE and viscosity profiles and a substantially uniform refractive index profile in an axial direction; heating an end of the preform to a temperature sufficient to draw fiber therefrom; and drawing fiber therefrom. In an aspect of this embodiment, the method includes applying either a uniform or nonuniform tensile force to the fiber as it is being drawn. The tensile force is in the range of about 25 to 200 gm.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.